The basic prior art systems for providing supplemental respiratory gases have consisted of a source of supplemental gas at a requlated pressure which is connected to a cannula hose with outlets for dispelling the supplemental gas in one or both nares of a patient's nose. These systems deliver oxygen at a constant rate without regard to the patient's respiratory cycle. Thus, a flow of oxygen is provided constantly by these systems, not only during the inhalation portion of the respiratory cycle characterized by negative pressure at the nares and the drawing in of respiratory gases, but also during the exhalation portion of the cycle characterized by positive pressure at the nares and the expelling of gas from the in vivo respiratory system. This approach not only wastes supplemental gas which is vented to the ambient atmosphere during exhalation, but further, where the supplemental gas is oxygen, may create an oxygen enriched atmosphere about the patient's face which creates a danger of fire related injury. These problems related to continuous supplemental gas supply systems are aggravated by the fact that the exhalation portion of an in vivo system's respiratory cycle is commonly of a longer duration than the inhalation portion of the cycle, thus wasting a larger portion of the supplemental gas.
Demand systems are known in the prior art which utilize a dual hose cannula and pressure sensing device to provide supplemental gas only during the inhalation portion of the respiratory cycle characterized by negative pressure at the nares, or point of inhalation by the in vivo system. Typically, in such systems, a first hose of the two hose cannula is dedicated to communicate a pressure sensor with a nare so that the sensor may sense positive and negative pressures at the point of inhalation which are associated with the respiratory cycle. The pressure sensor is then used to control the flow of supplemental gas from a constant pressure gas source into the second hose of the cannula and on to the second nare to provid a flow of supplemental gas only during periods of negative pressure characterizing the inhalation portion of the respiratory cycle. While such systems may be effective in reducing waste of the supplemental gas and reducing oxygen concentrations in the local atmosphere, double hose cannulas are cumbersome and expensive in comparison to the single hose variety, thus increasing the expense and reducing the utility of such systems.
Other demand systems of the prior art have attempted to utilize a single hose cannula to both communicate a pressure sensing device with a nare at the point of inhalation of the in vivo respiratory system and to provide supplemental gas to the in vivo system. Generally, in these systems, the pressure sensing device is first connected to the cannula to monitor pressure within the hose. Upon detection of a negative pressure conveyed through the cannula hose to the pressure sensor, indicating the initiation of inhalation at the beginning of a respiratory cycle, supplemental oxygen under pressure is introduced into the cannula for a predetermined period of time. In these systems, the time interval over which supplemental gas is provided during each respiratory cycle must be pre-determined, and cannot be controlled by the occurrence of positive pressure characteristic of the exhalation portion of the respiratory cycle. This is because the residual local high pressure of the supplemental gas introduced into the cannula hose, which results from the pressure gradient along the length of the hose associated with the flow of supplemental gas to the nare, masks the cycle indicative pressures which would otherwise propagate through the cannula hose from the nare. Thus, in single hose systems, the length of the oxygen supply interval is typically set manually at a constant value.
Alternatively, in these systems, supplemental gas is not introduced during selected respiratory cycles so that cycle indicative pressures in the cannula hose can be monitored and the duration of the inhalation portion of a sample cycle determined. The duration of the sample cycle is then utilized as a basis for setting the duration of one or more subsequent supplemental gas supply intervals. Concomitant with this latter approach is a reduction in overall supply of oxygen which, in turn, generally results in a lower concentration of supplemental gas in the blood stream of the patient than would result from supplying supplemental gas during the inhalation portion of every respiratory cycle.
Further problems occur in these prior art single hose cannula systems due to their valving and switching arrangements. Typically, the single hose prior art devices utilize a two-position three-port valve with a first port connected with the interior of the cannula hose and the second and third ports connected to the supplemental gas supply and the pressure sensor, respectively. In one position, the valve will connect the first port with the second port and in an alternative position, the valve will connect the first port with the third port. A control device switches the valve between these two positions so that, at any time, either the pressure sensor or the supplemental gas supply is in communication with the cannula hose. Difficulties with such systems arise where pressure transducers or piezo-electric pressure detection devices are utilized to measure pressures within the cannula hose because, when the position of the three-port valve is changed from the gas supply position to the pressure sensing position, a high residual pressure is present at the location of the first port in the cannula hose for a time after the switch due to the pressure gradient along the hose associated with the flow of supplemental gas to the point of gas introductin at the nare. These high pressures often cause the calibration point of pressure sensors to drift excessively and may damage the pressure sensors.